run: document positional args

Author: cirosantilli

README.adoc

@@ -9499,6 +9499,8 @@ Otherwise, it becomes very difficult to keep everything working across path refa
 ./build [OPTIONS] [-- EXTRA_MAKE_ARGS]
 ....
 
+`EXTRA_MAKE_ARGS` gets appended to the end of the `make` command line.
+
 ===== build configuration options
 
 [options="header"]
@@ -9558,6 +9560,8 @@ Otherwise, it becomes very difficult to keep everything working across path refa
 ./run [OPTIONS] [-- EXTRA_RUN_ARGS]
 ....
 
+`EXTRA_RUN_ARGS` gets appended to the end of the emulator command line.
+
 [options="header"]
 |===
 |Name   |Argument name |Description